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Difficulties understanding some basic concepts of thermodynamics

  1. Nov 13, 2006 #1
    Hi all,

    I m having some difficulties understanding some basic concepts of thermodynamics:

    -What is reversibility?

    Ok, i understand that if after the "go" and "return" processes, the system as well as the surroundings are back to their initial state, then we can say that the overral process was reversible.

    But is there any exemple of this?even theoretic?

    -Why is maximum work obtained when working in reversibility conditions?

    Thank you.
    Hope someone can help me.. :shy:
     
  2. jcsd
  3. Nov 13, 2006 #2

    russ_watters

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    An insulated piston compressing air is a reversible process. You'll find in thermo that all the cycles are broken down into pieces and each piece of the cycle has different attributes. Some will be reversible and some won't. A process (such as the compression I just described) without heat gain/loss is called adiabatic and adiabatic processes are reversible precisely because there is no heat gain/loss to the environment.

    If a process is reversible, there is no entropy - no heat lost to the environment. Heat lost to the environment decreases the useful work that can be done from the input heat.
     
  4. Nov 13, 2006 #3

    Andrew Mason

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    A reversible process is one that can be reversed with an infinitessimal change in conditions. It is an ideal that can be approximated but never achieved.

    A reversible flow of heat is a flow that occurs due to an infinitessimal temperature difference.

    An adiabatic compression or expansion may or may not be reversible. An adiabatic compression will be reversible if it is done by applying a pressure that is always an infinitessimal amount higher than the internal pressure of the gas. An adiabatic expansion will be reversible if it is done if the gas pressure is always an infinitessimal amount higher than the external pressure on the gas.

    AM
     
  5. Nov 13, 2006 #4

    russ_watters

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    AM, maybe you are talking about the practical vs theoretical, but being reversible is the definition of "adiabatic". So it isn't correct to say an adiabatic process may or may not be reversible: if it isn't reversible, it isn't adiabatic.

    This question is a Thermo 101 question, so there is no need (in fact, it can add confusion) to get away from the simplifications of the issues you get at that level.
     
  6. Nov 14, 2006 #5
    i thought that a process is reversible if the process could be brought back to the original state....ie entropy is 0. russ is correct. theoretically, with a perfect insulator, it is true that adiabatic processes are reversible all the time.

    but in the practical sense, no one has got a perfect insultor.
     
  7. Nov 14, 2006 #6
    Well, first of all thanks for replying.
    I get it a little better now, but still not good:

    1) "A reversible process is one that can be reversed with an infinitessimal change in conditions. It is an ideal that can be approximated but never achieved."

    -This is also the definition in my book.
    I read it over and over again and it just means nothing to me:
    Why would such a process be reversible?

    2) would you all agree that:

    "processes are reversible if there is no heat gain/loss to the environment during the "go" and "return" processes?"

    Because this definition kind of makes sense to me, in the way that, if there is now heat produced/lost, then total entropy variation is DS=0 and since heat is the less organised form of work available(and correct me if i am wrong: irrecuperable) then we can say that there was no energy lost while doing work, in a irrecuperable form.

    --->Is that correct?should I learn it that way?
    (if there is any error, please discuss it).

    3)Reversibility, irreversibility and lost opportunity to do work:

    Suppose we have a thermally insulated cylinder that holds an ideal gas.The gas is contained by a thermally insulated massles piston with a stack of many small weights on top of it.Initially the system is at mechanical and thermal equilibrium.

    consider the following three processes:
    -1-All of the weights are removed from the piston instantaneously and the gas expands until its volume is increased by a factor of four.
    -2-Half of the small weights are removed and the system is allowd to double its volume, then the remaining half are removed from the piston and the gas is allowed to expand unti its volume is again doubled.
    -3-Each small weight is removed from the piston one at a time, so that the pressure inside the cylinder is always in equilibrium with the weight on top of the piston.When the last weight is removed, the volume has increased by a factor of four.

    ---->>>>According to the authors of this, "Maximum work (proportional to the are under the curve in a ,force-volume graph,)is obtained for the quasi static expansion=3process described"

    It does look like the third process has the "maximum area under the curve", but, to my mind, in the first two processes, work has gone part into giving our piston a kinetik energy which in not the case in a "one at a time" removing weights process.
    So to my work is at least equal(if no heat was produced...)

    -Now, where am I wrong?I just dont get this and seems to be the most explicit example..:grumpy:
     
  8. Nov 14, 2006 #7
    adiabatic

    Adiabatic means a process where no heat is exchanged between the system and its surroundings
    An adiabatic process may be reversible or irreversible.

    A reversible adiabatic process is also called isentropic, meaning that the entropy of the adiabatic system does not change for such a processs.

    During an irreversible adiabatic process of a closed system, the entropy of the system increases. This is the preferred form of the Second Law of themodynamics....
    See my web page and reference to my texbooks

    http://thermochimique.epfl.ch/livres/ThermoE/
     
  9. Nov 14, 2006 #8
    I have done in my book an example of a process of a system in contact with a heat source. The change is carried out in one step or 2 steps and more.
    This is the objects problem 3.3 which is also corrected in my textbooks.

    I calculated the work done of the system in each case.

    I drew a graph that shows that the more steps, the closer you get to a reversible process.

    To help you out a bit, a reversible process is a process which is a continuous sequence of equlibrium states.

    http://thermochimique.epfl.ch/livres/ThermoE/
     
  10. Nov 15, 2006 #9

    Andrew Mason

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    The adiabatic process is reversible only if the work done by the gas in expanding is equal to the work done on the gas in compression. So long as the only criterion for "adiabatic" is that there can be no heat flowing into or out of the system, adiabatic need not necessarily be reversible.

    A free expansion of a gas into a vacuum in an insulated chamber is considered to be an adiabatic process but it is not reversible. It cannot, with an infinitessimal change in pressure, contract to its original volume using only the work that it did while expanding (because it did no work on the surroundings while expanding into the vacuum). The subtlety here is that there is work done by the gas on itself - which produces kinetic energy of the gas during the rapid expansion which then ends up as heat in the gas.

    The would prefer a different definition of adiabatic. But it seems I have lost that debate. See this thread, for example.

    In a free expansion of a gas, the gas does work on itself giving the gas molecules increased kinetic energy all in one direction (ie into the vacuum). If the gas is confined to a chamber, the gas molecules slam into the end wall and bounce around, adding heat energy to the gas. I would prefer to call this an adiabatic free expansion (that is isentropic) followed by a flow of heat to the gas (when the useable kinetic energy of the gas molecules becomes unuseable heat). But, as I say, I am a lone voice on that.

    AM
     
  11. Nov 15, 2006 #10

    russ_watters

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    Sorry about that - I forgot that peculiarity of the definitions and the dictionary I looked at had it wrong. You're right: isentropic is adiabatic, but adiabatic is not necessarily isentropic.
     
  12. Nov 17, 2006 #11
    Well, thank you all so much for replying!

    Last question:

    Have you got any internet sites to recommend to me so that I can read more about thermodynamics?

    Thanks!
     
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